Wind turbines are used to produce electricity. The trend goes to units with a large rotor diameter. The Growian in Germany has a diameter above 100 meters. The new largest concept goes up to 175 meters. It turned out that production costs for this kind of wind electricity energy generators are so high that the period of amortization is longer than the expected life time of the machine.
The invention shows a way to produce electricity in an economical way.
The invention shows, how powerful wind machines can be made without large rotors. The solution is a multi-rotor tower. If conventionally designed wind turbines are analyzed, it is apparent that all types have some common disadvantages than can be eliminated by using small rotors. The following compares conventional 54-ft diameter wind turbines and multi-rotor turbines with four rotors of one half the diameter. The diameter of 54-ft was chosen because more than 3,000 wind machines with this rotor diameter have already been sold. In contrast to that trend it can be proven, that large rotors show much higher cost per KW than a group of small rotors together having the same total swept area and performance. Transformation of wind power into mechanical performance takes place within the plane of the rotor, therefore the rotor is the most important element of any wind turbine.
A rotor assembly consists of the rotor hub and blades. In the case of a rotor without pitch control, which shall be compared in this case, about 80% of all costs are within the blades. The price of blades with the same mechanical properties depends on their size. Material costs are proportional to weight. The weight of a blade follows the equation: EQU G=G.sub.o .multidot.L.sup.3
L=Length of blade PA1 .omega..sub.f =Angular velocity PA1 W=Weight of blade l=Length of blade PA1 f=frequency PA1 I=Moment of inertia PA1 W=Weight of blade PA1 D=diameter of rotor PA1 TRQ=Torque on shaft PA1 z=Number of blades PA1 H=height [ft] PA1 V.sub.33 =Wind velocity in 33-ft PA1 .sigma.=max. stress PA1 M.sub.B =bending moment=P/V.multidot.H PA1 W.sub.B =I/C=section modulus PA1 H=tower height
When comparing the two rotor systems, this equation shows a weight ratio of 8:1. Four small rotors produce the same energy as one large rotor. Therefore, material cost for four small rotors is only 50% of the material cost for one large rotor. The real savings are considerably higher because the production of one large rotor needs much more investment for jigs, tooling and labor than production of four sets of blades for small rotors. Therefore, also investment for production means and labor costs lie far below comparable costs for large rotor and blade assemblies.
The invention shows, how powerful windmachines can be made without using large rotors. The solution is the use of a multi-rotor tower.
Upwind rotors are exposed to the undisturbed air stream. The costs of upwind turbines are higher than for downwind turbines because the upwind machines require sensitive high torque servo drive means for the heavy duty yaw control bearing systems. The disadvantage from the engineering standpoint is the fact that, as each blade crosses the front of the tower, cyclic forces excite cyclic vibrations causing fatigue in the tower structure. Therefore, upwind type tower construction requires more material. The engineering advantage of downwind wind turbines is that they do not require mechanical means for yaw control, however, the blades crossing the wind shadow of the tower, undergo extreme cyclic excitations. This is one of the main causes for blade distruction by fatigue.
The multi-rotor system according to the invention combines the advantages of upwind turbines, e.g., the undisturbed wind stream, with the advantage of the downwind turbines, e.g., the automatic yaw control--without the above described disadvantages of single rotor wind turbines.